All conventional image forming devices are designed to form images on printable media that are fed into the image forming devices either by external feeding devices coupled to the image forming devices or by certain internal feeding mechanisms incorporated into the image forming devices. Ordinarily, the printable media has at least one side of its surface suitable for forming information-carrying images thereon. However, many commercially available printable media have printable surfaces on both sides. There are many types of printable media available in the market, such as regular papers, labels, or thermal paper, etc. Thus, every conventional image forming device is adapted to receive at least one type of printable medium for printing purposes. In addition, many different types of image forming devices, e.g., printers, fax machines, or copiers, etc., are also available in the market, and the methods these image forming devices use to impart images on the printable media are often very different. For example, some image forming devices need ribbons to transfer images to the printable medium, while others use a direct thermal transfer method to form images on the printable medium.
Moreover, even one type of commercially available printable media may have many different forms. For instance, the printable media may come in the form of separate sheets, or it may be in the form of a continuous paper strip. Consequently, different types of feeding mechanisms are specially designed and are adapted to be incorporated into selected printers for respectively feeding these different forms of printable media into the printers. Separate sheets of plain paper are undeniably still the predominant printable medium type that is used in today's business applications. There is, however, an ongoing need to have a printable medium in the form of a strip wrapped into a printable medium roll. Typically, this printable medium roll, such as a label roll, has the printable medium strip, such as a label strip, winding around a cardboard support tube, or a similarly shaped tube made by other suitable materials. In the case of a label roll, the label roll has serially arranged labels positioned on the label strip, and each label of the label strip has a printable surface on a front side and an adhesive back side attached to a continuous protective backing of the label strip. The protective backing of the label strip generally has a treated glossy surface attached to the adhesive side of the labels permitting the labels to be easily peeled off from the protective backing. The labels are then, together with the protective backing strip, wound around the support tube to form the label roll.
Most conventional printers have relatively simple holding mechanisms incorporated therein for holding label rolls. Typically, a conventional printer includes either a cylindrically shaped roll holder transversely positioned across the printer or, alternatively, two ear-like spool ends positioned at corresponding opposite inner sides of the printer. As a result, the label strip in the printer normally moves in a forward direction, which is perpendicular to the axis of the label roll, toward a front side of the printer. The roll holder, or the pair of spool ends, of the conventional printer function to hold the label roll in order to facilitate the label strip to be unwound and moved toward a print head of the printer. Each conventional printer has a print head, and the print head is the part of the printer that converts electrical signals into images formed on the printable medium, such as the label strip. Additionally, a platen is rotatably mounted within the printer and is adapted to press the label strip tightly against the print head for receiving the images. The platen is coupled to a motor for rotation in order to move the label strip through the printer. In some conventional printers, their cylindrically shaped roll holders (and/or the ear-like spool ends, do not themselves rotate during operation of the printers. They work only as a stationary roll holder support for the printable media. In other printers, the roller holders or the spool ends are rotatable.
Most conventional printers, and other image forming devices that use printable medium rolls, often experience a common problem of loose media. The loose media problem happens when the printable medium strip traveling within the printer becomes loose before and/or after being fed through the print head of the printer. There are different reasons that may cause the loose media problem in conventional printers, but it is almost impossible to predict when or how often this loose media problem will happen to any particular printer. The loose media problem frequently causes printing errors, such as skid printing or double printing, on the printable medium. It may possibly jam the conventional printer as well.
The loose media problem generally occurs when various parts of the printable medium strip travel through the conventional printer at slightly different speeds. The speed differences experienced by different parts of the medium strip are normally due to the inability of the conventional printer to move the printable medium strip at a constant speed throughout the printer. For instance, the platen of the conventional printer may move the label strip at a first speed that is slower than, albeit maybe slightly, a second speed traveled at by the label strip moved by the roll holder. As a result, a section of the label strip between roll holder and the platen may become loose.
In addition, a user may often need to move forward and/or back up the printable medium strip in the printer in order to adjust the position of the printable medium strip within the printer. Moving forward or backing up the printable medium strip in the printer is ordinarily achieved by rotating, manually or automatically by a motor of the printer, the platen of the printer. After the platen has been rotated to adjust for a proper printing position of a particular label on the label strip, the label roll will also need to be adjusted accordingly to maintain the tightness of the label strip between the label roll and the print head. Otherwise, the label strip will become loose and may cause many printing problems, such as a sudden jerk or a jump, when the next printing job begins. A built-in mechanism of the printer is therefore needed to maintain tightness of the label strip, or of any other type of printable media used, within the printer to prevent the loose media problem.